The Facts of Light: How Many Solar Panels do I Need?

We get a lot of questions about solar power. That’s why we’ve introduced “The Facts of Light”—a place where you can inquire about all things solar, and we’ll do our best to get you the answers.

Today’s question is one of our most common.

Q: How many solar panels do I need?

Few people are more uniquely qualified to answer this than David Schonberg, SolarCity’s Director of Engineering. Today, SolarCity’s customers enjoy the cost-saving, clean-energy benefits of going solar, thanks in large part to Dave and his team.

According to David, the answer starts by asking two more questions:

How much electricity do you use at home?

How much electricity could solar panels on your home potentially produce?

So let’s take these one-by-one.

How much energy do you use at home?

To figure this out, just look at your electricity bill.

You should see the amount of electricity—measured in kilowatt-hours (kWh)—your home has used over the past year. You’ll probably see the consumption rise in the summer months (you’re running the air conditioner more frequently) and drop in the winter.

That annual consumption is David’s target.

In a “perfect” situation, a solar installation would provide enough power for all the home’s electrical needs. In reality—for reasons we’ll discuss in the answer to the second question—this target isn’t always possible. However, an installation that provides even a fraction of a home’s electrical needs can save a homeowner significant money over the course of several years, and can make a serious impact on that home’s carbon footprint.

How much electricity could solar panels produce on the home?

The answer to this question derives from several factors, including:

Your geographic region—This sets a baseline for how much sunlight can hit your roof. We gauge the amount of sunlight a region receives with a value called “solar insolation”. Some regions simply get more sunlight than others.

The output of the solar panels—This is measured in terms of wattage. Currently, SolarCity typically installs either 260 or 265-watt panels. This means that when installed and exposed to the sun, the panel will produce up to that amount of wattage.

The orientation of your roof—Solar panels work best when they’re facing the south, east or west, at an angle between 15 and 30 degrees (which happens to be the “pitch” of most roofs), and of course, with a direct line of view to the sun. The size of your roof—The larger the roof, the more panels we can consider installing. A little measuring and simple math helps us determine the maximum number of panels we can install.

As you see, when estimating the size of a solar installation, engineers like David have a few things to think about.

Looking at the question a slightly different way

Every home, and every situation, is unique. The fact is, a solar installation can provide significant financial and carbon benefit even in “less than optimal” conditions. A home in the high desert of California can benefit from going solar … as can a home on a tree-shaded street in Massachusetts.

So the ideal question might not be “How many panels do I need?” but “How many panels can benefit me?”

To help you get a ballpark idea, the average SolarCity system is around 6kW (kilowatts) and consists of 24 panels. Production for this system will vary slightly depending on your geographic location: a 6kW system in California will produce approximately 1,400 kWh/kW, whereas the same system will produce 1,600 kWh/kW in Arizona, since that region receives more sun. A 6kW system in the Northeast, which sees less sun on average, can be expected to produce 1,200 kWh/kW.

To estimate how many panels your home would need, divide your total annual energy consumption by the regional production most relevant to you (1,200, 1,400 or 1,600 kWh/kW, depending on where you live) and multiply by 4 (the approximate number of solar panels needed to produce a kW). This number could go up or down depending on shading and the direction of the panels based on your roof.

The easiest way to understand what size system can benefit your home is to contact SolarCity for a free consultation. A quick call and answers to a few questions gets you started.

A solar power system is customized for your home, so pricing and savings vary based on location, system size, government rebates and local utility rates. Savings on your total electricity costs is not guaranteed. Financing terms vary by location and are not available in all areas. $0 due upon contract signing and no security deposit required for leases and PPAs. A 3 kW system starts at $25-$100 per month with an annual increase of 0-2.9% each year for 20-30 years, on approved credit. SolarCity Corporation will repair or replace broken warranted components. SolarCity DBA Tesla Energy CA CSLB 888104, MA HIC 168572/EL-1136MR, other contractor licenses. SolarCity is not the lender and only the third party lender may approve, offer, or make a loan.

Savings based on SolarPPA and SolarLease customers with at least twelve months of billing data. Savings Rate calculated by subtracting PPA or equivalent lease kWh rate from relevant utility kWh rate. Savings calculated by multiplying actual kWh supplied by SolarCity in customers' first year times Savings Rate. Excludes fully or partially prepaid contracts.

Solar Bonds are debt securities issued by SolarCity. As with any investment, purchasing Solar Bonds involves risk. You must make your own decision about whether and how much to invest in Solar Bonds. SolarCity cannot make any investment recommendations or otherwise provide any investment advice. SolarCity has filed a registration statement (including a prospectus) with the Securities and Exchange Commission (“SEC”) for offerings to which information on this web site relates. Before you invest, you should read the prospectus in that registration statement and other documents SolarCity has filed with the SEC for more complete information about SolarCity and the offerings. You may get these documents for free by visiting EDGAR on the SEC web site at www.sec.gov. Alternatively, you may obtain the prospectus relating to the Solar Bonds, and the pricing supplement relating to a particular series of Solar Bonds, at solarbonds.solarcity.com.

*Based on SolarCity average system size of 6 kW and 8,418 kWh average first year production degraded by .5% annually over 30 years. Environmental benefits based on data collected from: Environmental Protection Agency, US Geological Survey, Global ReLeaf, and National Geographic April 2014.